I think for once I might be on Microsofts side.
They don't promise that there will be 99.9% uptime, instead they are very clear in that they are providing a guarantee to refund you money if they fail to make 99.9% uptime.
Calling it Office 365 means that averaging over four years they can achieve 6 hours down time a year and still have a days average exceeding 365, alternatively have 100% uptime for 3 years and then on the leap year take it down for Feb 29th.